1. Field of the Invention
The present invention relates to software for use with the PostScript language in the production of multicolor images for prepress color proofing. Black-and White PostScript files are used to generate multicolor images useful for printing.
2. Background of the Invention
One of the most difficult aspects of simulating a printed image in a prepress color proof which does not actually print images from plates is the reproduction of mechanical phenomena or events that occur on press and which affect visible characteristics of the printed image. One of the more traditional phenomena which must be simulated is "dot gain," in which the half tone dots are physically spread out during printing, so that the size of the dot on the receptor sheet is not the same size as the dot on the plate. Dot gain varies according to the size of the dot, the type of press used, and the inks used, so that a proofing system must attempt to adjust for each of these variables which affect dot gain. That is no easy task.
Another event which takes place during printing is color trapping. Color trapping is necessary in the design of almost all full color pages. Trapping is a method used in the printing industry to allow for the slight misregistration of colors that occurs on the printing press. When two dissimilar colors are printed side by side, a thin area of one color is extended into the other color. If the printing press were to misplace the one color slightly away from the other, the trapping will prevent the white paper from showing through the gap.
Printing inks are transparent, so the small area where the two colors overlap will result in a darker color. The human eye is much less sensitive to this darker area than to a white gap.
The PostScript language is conventionally used to draw the text, line work, and color areas describing a printed page in many black and white, and color printers used in the graphic arts industry as well as imagesetters which create film for printing plates or proofs to check the accuracy of the page designs.
When making film for printing plates, PostScript is quite able to reproduce the color trapping needed for high quality pages. Since a separate printing plate is needed for each color, the PostScript file will draw a complete and separate page for each color separation. Each color would be represented by a single separation page.
When making a color proof, PostScript presently is not able to reproduce the color trapping on the proof. This is because PostScript considers all colors to be opaque, not transparent. This means that where two colors are to overlap on the digital data, one color will be drawn slightly underneath another color. However, when a color is drawn in the PostScript color file over the top of the other color, the area underneath the top color will be completely obliterated in the data. Thus, even though the original data of the page has proper trapping, the user will not be able to see the trapping on the color print. A page with trapping and a page without trapping will appear identical when generated by a normal color PostScript printer.